In the electronic or micromechanical components industries, machining or plasma treatment processes are used that are performed in an enclosure where it is necessary to maintain a controlled vacuum atmosphere.
Generating a vacuum requires pumps to be used that are capable of generating quickly a high vacuum that is suitable for the machining or treatment process, and that are capable of maintaining it. In general, turbo/drag type pumps are used, comprising a pump body in which a rotor is caused to rotate rapidly, for example rotation at more than 30,000 revolutions per minute (rpm).
With such a high speed of rotation, the rotor acquires very high kinetic energy, and is subjected to high mechanical stresses which require suitable materials to be selected.
The rotor of a turbo/drag vacuum pump is constituted by a segment of the rotor that is upstream (in the gas flow direction) and that has turbine type blades, and a segment of the rotor that is downstream (in the gas flow direction) and that is in the form of a Holweck type skirt.
In the description and the claims, the terms “upstream” and “downstream” designate respectively those portions of the vacuum pump that are passed through initially and finally by the gas pumped in the direction in which the gas flows in operation.
The upstream segment having turbine type blades is complex in shape, and is made out of a suitable metal such as aluminum or an aluminum alloy. Its shape is too complex to enable it to be made economically out of composite material.
The downstream segment, in the form of a Holweck type skirt, is a thin wall in the form of a body of revolution, largely cylindrical in shape, and driven to rotate in a downstream segment of a stator having helical grooves of progressively tapering section.
At present, the pumping performance of turbo/drag pumps at high speeds of rotation is limited by the fact that it is not possible to increase the diameter of the Holweck skirt beyond a maximum limit. A priori, it is known that it is possible to increase pumping performance by increasing the diameter of the Holweck skirt. However such an increase turns out to be impossible to achieve while using conventional materials, in particular metals, or even composite materials based on a metal matrix and containing reinforcing additives such as ceramics, powders, or fibers of carbon or other reinforcing materials. The highest mechanical stresses appear in this region of the rotor and they are proportional to the density of the material constituting the skirt, to the square of the speed of rotation of the rotor, and to the square of the diameter of the rotor.
In order to reduce stresses in the Holweck skirt, it is necessary in particular to reduce its mass. To do this, proposals have already been made for rotors in which the downstream segment in the form of a Holweck skirt is made of an organic matrix composite material based on fiber-filled resin. That solution provides the advantage of using a material having better mechanical properties. The downstream segment is connected to the upstream segment via an annular connection region. In this annular connection region, the organic matrix composite material constituting the Holweck skirt is secured to the upstream segment which is made of metal.
However, a difficulty then lies in the differences between the mechanical and thermal properties of the organic matrix composite material constituting the downstream segment of the Holweck skirt rotor and the corresponding properties of the metal or alloy constituting the upstream segment of the rotor. Because of these different properties, large mechanical stresses appear in the annular connection region while the pump is in use, i.e. while the rotor is rotating rapidly in the presence of a rise in temperature due to the pumped gases being compressed. These mechanical stresses lead to weakness of the connection region and to a risk of rupture. Thus, the diameter of this connection region cannot be increased too much.
Conversely, if an organic matrix composite material is used that has mechanical and thermal properties that are more compatible with those of the metal constituting the upstream segment of the rotor, thereby in particular obtaining flexibility capable of accommodating deformation under stress, then the mechanical properties in the downstream region of the Holweck skirt are no longer sufficient to withstand the stresses that need to be supported during high-speed rotation of the rotor.